Even if this is true. What you might be observing is the effect not the cause.
In other words ... older people that are healthier are able to exercise more. Just a thought.
Statistical effects can really mess with people's perception of cause and effect.
"Survivorship bias or survival bias is the logical error of concentrating on entities that passed a selection process while overlooking those that did not."
A classic example of something like it, is during WW I the British saw a lot of airplanes were coming back with fuselage damage. So they started reinforcing the fuselage. Why this is a logical mistake is that these were the surviving airplanes.
It's both. If you don't exercise at all you will waste away - the older you are the worse this gets. But I know two people a little over 80. Both were very active people up to about 5 years ago when one got arthritic hip and knee issues for which there is no easy fix. They are nearly debilitated and this is affecting the rest of their health. The second person just keeps on going. The first person would definitely be the same if not for the issues.
Note that they both made it to 80. Presumably because they were continuously active until that age at least.
That reminds me of my father (currently 76) who got a large dog. Now he walks it (or the dog walks him) for miles twice per day and his doc is extatic because that got rid of quite a few of his old age problems.
"Survivorship bias or survival bias is the logical error of concentrating on entities that passed a selection process while overlooking those that did not."
A classic example of something like it, is during WW I the British saw a lot of airplanes were coming back with fuselage damage. So they started reinforcing the fuselage. Why this is a logical mistake is that these were the surviving airplanes.